Materials that are in the "public domain" are works that are not protected by any kind of intellectual property laws, including copyright. Anyone may use public domain works without seeking permission from or compensating the copyright holder, including for commercial purposes.
The phrase "public domain" actually does not appear anywhere in the U.S. Copyright Act, but is commonly accepted to mean that these works are no longer owned by any one individual but rather owned by the public.
There are several ways that a work can enter the public domain, including:
Under U.S. law, copyright protection only lasts for a certain length of time. That length of time is determined by the copyright laws set by Congress, which have changed over the years.
Determining when copyright expires on a work can be tricky and should be done on a case-by-case basis. Additionally, international copyright laws may differ and a work produced outside the U.S. may have a different copyright status.
Copyright does not apply to all kinds of works. In order to be protected by copyright, the work must be original, creative, and fixed in a tangible form. This includes "literary, dramatic, musical, and artistic works, such as poetry, novels, movies, songs, computer software, and architecture." (copyright.gov)
Copyright does not protect the idea behind a work or non-creative works, such as data or facts. Copyright also does not protect works that have not been fixed in a tangible form, such as a speech that hasn't been previously written or recorded in any way.
Example: Data are considered "facts" and therefore are not copyrightable. This includes the form in which the data are described. In order to be protected by copyright, the data visualization must demonstrate considerable creativity, such as an infographic. By that logic, this bar graph is not protected by copyright.
It's important to note that even if a work is not protected by copyright, it could be protected by another kind of intellectual property law, such as a trademark or patent.
Any work created by an employee of the U.S. federal government is not protected by copyright. However, copyright protection could apply if the work was created by a consultant or freelancer. Copyright protection may exist for works created by other federal governments, such as Canada.
Any creator of a copyrighted work may choose to waive their copyright and place their work in the public domain, using the Creative Commons CC0 option.